The sleep cycle in an important part of dreaming. In the sleep cycle there are four stages before Rapid Eye Movement (REM).
Researchers have been able to identify the different stages by monitoring people's eye, brain waves, and muscle movement while
they sleep. It takes approximately ninety minutes to go through the first four stages of the sleep cycle before going into
REM sleep.

The first stage is a light sleep before moving onto the second stage. In the second stage, images start to appear in your
mind. When in stage three, the heart rate slows down, muscles become more relaxed and blood pressure begins to fall.
Also, breathing becomes an even and steady pace. Stage four is when dreaming begins. It contains the deepest
sleep out of all stages. In stage four, it's difficult to wake a person. Their heart rate, blood pressure, and brain activity
increase and leads them into REM sleep.(Schulze)

(PSY255; Introduction to Biopsychology Edward I. Pollak, Ph.D.)

EEG Map of the Sleep Stages

Click on this picture to get a full size view of EEG's in the different sleep stages.

There are some people who skip over REM once their cycle begins for the second of thrid time. By skipping over REM, this shows
that the person either has a sleeping disorder or a large emotional disturbance(Cohen). REM sleep is important to our memory,
learning, and restoration. People who don't have enough REM sleep are often anxious, irritable and depressed(Schulze).

REM sleep is where the most brain activity occurs because this is the stage you have the most dreams in. When someone is in
REM sleep, they often appear paralyzed. Although their muscles don't move, their eyes do. While looking at someone in REM
sleep, you can see their eyes moving under their eyelids(Schulze). An electroencephalogram (or EEG) is a diagram of a persons
brain waves(Farlex). When someone is in REM sleep, their EEG's are extremely similar to a person who is awake but sitting
still. This indicates that they have around the same amount of brain activity occuring(Schulze). EEG's are measured in beta
waves which are a wave form of the adult brain. Beta waves generally have a frequency from 12 to 30 hertz(Hz)(Farlex). In
REM sleep, a person's EEG's has beta waves of 14 to 30 Hz. Out of REM sleep, a person has EEG's of 8 to 13 Hz. This shows
the drastic difference in brain activity when in and out of REM sleep. Also, if someone is woken up while they're in REM sleep,
they will be able to recall a large portion of their dream.

The cycle of the sleep stages repeats itself up to five times throughout the night(Schulze). Each time it repeats itself
the REM stage lengthens. Where it may have only lasted for 45 minutes the first time, it can last an hour or an hour
and a half the second time it cycles around. This means that the longest dream a person has is more likely to occur in the
early hours of the morning.